A wide variety of propylene, homo and copolymers, both crystalline and amorphous, are commercially produced in gas phase or slurry phase processes. The use of high activity catalyst systems in both processes results in polypropylene resins with low levels of catalyst residue and low levels of amorphous polymer fractions. These low levels of catalyst residue and amorphous polymer fractions eliminate the need for costly extraction and catalyst removal steps. These high activity catalyst systems generally include a Ziegler-Nata type polymerization catalyst that is a magnesium chloride supported titanium tetrachloride; a co-catalyst that is an organoaluminum compound such as triethylaluminum, and an electron donor.
U.S. Pat. Nos. 4,547,476; 4,829,037; 4,839,321; 4,847,227; 4,970,186; 5,066,737; 5,077,357; 5,082,907; 5,106,806; 5,122,494, 5,124,298 and 5,130,284 disclose what is known as fourth generation catalyst systems. These fourth generation catalyst systems typically employ a solid component that is magnesium chloride supported titanium tetrachloride catalyst containing an internal electron donor, in combination with an organoaluminum co-catalyst, and what is known as a selectivity control agent (SCA) that is a silicon compound such as n-propyltrimethoxy silane, phenyltriethoxy silane, or cyclohexylmethyldimethoxy silane.
The very high-activity of the fourth generation catalyst systems results in very low levels of catalyst residue. However, it has been discovered that the very high-activity of these fourth generation catalysts systems causes problems in gas-phase fluidized bed reactor processes in that the fourth-generation catalyst systems are hard to control or kill. When the reactor needs to be idled for maintenance, something must be done to stop the polymerization process prior to stopping the flow of material out of the reactor or the reactor will fill with solid chunks of polypropylene. Catalyst poisons such as carbon monoxide or carbon dioxide could be introduced into the gas-phase reactor to kill the fourth generation catalyst system. However, the introduction of catalyst poisons is costly because the poisons must be purged from the reactor before the polymerization can be revived.
It would be very desirable, while performing maintenance or repairs on the reactor unit, to be able to temporarily slow or kill the activity of the fourth generation catalyst system in a process employing gas-phase fluidized bed reactors and to easily revive the reaction once the repairs have been completed.